Well, I don't know if my way is the most absolute correct method, but here's what i've noticed.

Vehicles should be centered at 0,0,0 in 3Ds Max and then in carcass just leave it at 0,0,0. That 24 or -24 that Duncan's tutorial says shouldn't be necessary.

Mknote, my guess would be that your bounding box is not set up correctly. You probably used the Tie Fighter's .veh and .npc files as a base. The Tie Fighter has a very high height parameter.

All vehicles need to have their bounding boxes set manually in the .veh and .npc files. The bounding box will extrude equally from the center axis or 0,0,0. For example, if you set the height parameter at 200, then the bounding box will go up 100 from 0,0,0 and down 100 from 0,0,0. The same thing happens for the length and width parameters as well.

One easy way to get a start on what measurements to use for your vehicle is to load it up in ModView and then select the show bounding box option. This will show you the measurements of each axis. You can then input these numbers into the corresponding fields in the vehicle file for height, length, and width. Also, make sure the height parameter in the .npc
ile is the same as the height parameter in the .veh file. Don't worry about the other number in the .npc file. Mike G of Ravensoft told me once what that number is for but I don't remember, either way it never seemed to important to mess around with so I don't.

Then load up the game and spawn your vehicle. Get in the vehicle and then type cg_showvehbounds 1 in the console. This will allow you to see the actual bounding box. Now if you spawned the vehicle at an angle it will not appear correctly so straighten the ship out. For some reason bouding boxes only rotate in certain degree increments.

Then all you gotta do is go back into the .veh file and .npc files and fine tune the settings until it meets your satisfaction. Most like the height parameter is the one you'll need to change the most.

If the bouding box isn't set up right not only will it cause the vehicle to either spawn in the ground or too high in the air, but it will cause collision detection to be off resulting in you being able to fly through things without being hit or be hit by things you're not even close too,Unfortunately, most user made vehicles i've seen have horrible bounding boxes, although I will say that a few certain vehicles do have shapes that are difficult to do due to the limitations of only having a big square cube for your bounding box.